1. Field of the Invention
The present invention provides a photo sensor, and more particularly, to a photo sensor applied to an embedded-on-glass ambient light sensor.
2. Description of the Prior Art
Flat display device has become one of the main components of mobile information products, such that the mobile information product manufactures and the display device suppliers have to continuously research on and design products with low power consumption. In order to achieve this goal, the mobile information product manufactures have designed an independent hidden-type photo sensor in the current circuit of product for sensing the intensity of ambient light of the display to finely adjust the brightness of the backlight of the display device, such as a liquid crystal display (LCD), by a program set in the information product, in order to save the power of the display device. However, the independent photo sensor disposed out of the display device has a disadvantage of large volume and poor accuracy of sensing result of the ambient light of the display device.
Currently, display device manufacturers also research on an embedded-on-glass ambient light sensor whose photo sensor is directly disposed on a glass substrate of the display device by adopting the low temperature polysilicon (LTPS) technique for replacing the conventional extra independent photo sensor and further saving the cost and total volume of the information products. Moreover, because the distance between the photo sensor and the LCD screen is very small (less than about 0.4 micrometers), the strength of ambient light can be accurately detected and the detecting photo current can be transformed into a steady output signal directly on the glass so as to adjust the backlight brightness according to various environments. As a result, the power of the battery of the end-product can be saved, and a better performance of the display image is supplied.
Referring to FIG. 1, FIG. 1 is a sectional-view of a photo sensor according to the conventional embedded-on-glass ambient light sensor technology. The conventional photo sensor 10 is disposed on the glass substrate 12, having a patterned shielding layer 14, a buffer oxide layer 16, a patterned semiconductor layer 18, a dielectric layer 20, and a planarization layer 22 from bottom to top. The patterned semiconductor layer 18 comprises a P-type doped region 24, an intrinsic region 26, and an N-type doped region 28 as an PIN diode, wherein the P-type doped region 24 and the N-type doped region 28 are respectively electrically connected to the corresponding contact elements 32 positioned above the dielectric layer 20 through the connection elements 30. When light illuminates the photo sensor 10, the intrinsic region 26 will be excited to form electron-hole pairs, resulting in photocurrent that can be outputted through the contact elements 32.
The sensitivity of the conventional photo sensor 10 can be obtained by measuring the photo current and the dark current, wherein the photo current represents the photocurrent formed under an illumination of ambient light, and the dark current represents the backflow current of the conventional photo sensor 10 under no illumination. The sensitivity is defined as the ratio of the photo current to the dark current. According to the measurement result of the conventional photo sensor 10, the ratio of the photo current to the dark current is far less than 100, thus the sensitivity of the conventional photo sensor 10 is not high enough when it is applied to information products. Accordingly, the variation or change of ambient light cannot be effectively detected by the conventional photo sensor 10, which looses the function of providing data for finely adjusting the backlight of the display device.
As mentioned above, the technology of current embedded-on-glass ambient light sensor still has the problem of poor sensitivity, so that it has limited performance when applying to mobile information products practically. As a result, the manufacturers still have to continuously research and develop photo sensors with high sensitivities.